In one aspect, this invention relates to an apparatus for gas treating a bed of particles. In another aspect, the invention relates to a method of gas treating a particle bed.
In many alternate fuels processes, heating, drying, retorting, and cooling of solids particles is accomplished by locating the packed solids on a large diameter rotating grate or long, straight conveyor system and by circulating a gaseous medium through the solid bed. A major disadvantage of this type of packed-bed system is that there is a large temperature gradient produced over the bed depth. In order to achieve the desired average bulk solids temperature over the entire bed depth, it is necessary to raise the particle temperature at the upper portion of the bed considerably above te average bulk solids temperature, i.e., often approaching the inlet gas temperature. For oil shale preheating, this can lead to undesirable, premature retorting, and for coal drying, this can produce detrimetal surface cracking and fines generation. To avoid this particle overheating, the inlet gas temperature level must be reduced, thereby reducing packed-bed heat transfer effectiveness. The large temperature gradient also results in longer residence time or moving bed length. An impractically long residence time in a particular heating region or a long bed length is necessitated in prior systems in order to heat the bottom layers in the bed to a level which accomplishes a desired average bulk solids temperature. Long residence times necessarily reduce the efficiency of a gas treatment system. The need to employ long beds requires utilization of very long (i.e. 700-800 ft.) or of large diameter (i.e. 250 ft.) grates which are highly expensive and difficult to properly manipulate.